As energy costs rise it is becoming increasingly necessary for industry to minimize these costs in order to become and remain competitive. One way of reducing energy costs is to use waste energy which is a byproduct of one activity in another activity. For example a heat exchanger can be used to receive two fluids and transfer heat energy from one of the fluids to another. Thus, instead of wasting the heat energy in the first fluid and using more energy to heat the second fluid, the same heat energy is used two or more times.
Such heat exchangers typically employ a plurality of heat exchange tubes which carry one fluid on the inside thereof and are exposed to the second fluid on the outside thereof. If the walls of the heat exchange tubes are made of a thermally conductive material such as copper or some other metal, heat readily passes through the walls of the tube from the warmer fluid to the cooler fluid. Typical examples of heat exchangers employing such tubes are shown in European patent application No. 66,425 A3 and European patent application No. 120,497. In each of these heat exchanger arrangements a plurality of heat exchange tubes are nested inside a container while first and second fluids are directed through the container to improve heat transfer efficiency.
Where heat from a toxic heat exchange fluid such as a refrigerant is to be exchanged with a consumable fluid such as potable water, safety or code requirements frequently demand the use of a multi-walled heat exchange tube. To be effective or comply with code requirements a gap must be maintained between adjacent tube walls so that if a leak develops in one wall the leaking fluid will flow through the gap to the exterior of the tube where it can be detected. At the same time, the second wall maintains the leaking fluid separated from the other fluid. For certain applications it becomes necessary to further improve safety by using a triple wall tube. When a triple wall tube is used, two heat exchange fluids can become mixed only if all three walls develop a leak simultaneously.
While the use of multi-walled heat exchange tubes significantly increases the safety of a heat exchanger, the efficiency of heat transfer between the two fluids is significantly impaired. The tubes themselves can be made of a relatively good heat conductive material. However, the gap between the tubes becomes a thermal insulator and significantly reduces heat transfer efficiency of a heat exchange tube. The heat transfer efficiency can be improved somewhat by swaging a helical groove into the outer tube over a substantial portion of the length of the tube. However, in order to maintain a sufficient gap between adjacent tube walls that provides communication with an open end of a tube so that a fluid leak can be detected, the groove must not be permitted to produce wall to wall contact between adjacent tubes over more than a small portion of the total surface area of a heat exchange tube.
This limitation on adjacent wall direct contact means the heat transfer efficiency remains low when compared to a single wall tube. As a result, either less heat is transferred between the two fluids, thus increasing manufacturing costs, or a larger heat exchange tube surface area must be provided, thus increasing the size and cost of the heat exchanger.
The present invention significantly increases the efficiency of multi-walled heat exchange tubes by inserting a liquid in the gap between adjacent tube walls. The liquid provides a heat transfer efficiency that is far superior to that of the gases that are normally found in the interwall gap. Since the liquid will be pushed out of the gap and detected in the event of a leak in any of the walls of the tube, the safety of the tube is not impaired. If one end of the tube is sealed, the liquid can be inserted by first evacuating the tube and then forcing the liquid into the gap. The seal at the closed end, coupled with the relatively small size of the gap, causes the liquid to be retained within the gap unless forced out by fluid leaking into the gap. It thus becomes possible to significantly reduce the cost of an industrial process by either reducing the heat exchange surface area or by increasing the energy transfer between two heat exchange fluids.